1. Field of the Invention
Aspects of the invention relate to a switching power source apparatus and a control method thereof, and more particularly to a zero voltage switching power source apparatus, and a control method thereof.
2. Description of the Related Art
A power source apparatus may be classified into a switching mode power source apparatus or a non-switching mode power source apparatus, such as a linear power source apparatus, depending on a DC voltage converting method. A non-switching mode power source apparatus has a low power efficiency, but a high stability. In contrast, a switching mode power source apparatus has a higher power efficiency, but a lower stability. Even though the switching mode power source apparatus has a lower stability than the non-switching mode power source apparatus, its higher power efficiency, coupled with recent improvements in stability, have resulted in an increase in the use of the switching mode power source apparatus in circuits.
A switching mode power source apparatus may be operated in a zero voltage switching mode, in which a switching operation is performed only when a voltage across the switch is zero, thereby minimizing a switching loss.
FIG. 1 is a detailed circuit diagram of a zero voltage switching power source apparatus according to the related art. As shown therein, a zero voltage switching power source apparatus 1 according to the related art includes a MOSFET (metal-oxide-semiconductor field-effect transistor) having a fast switching speed and having a diode connected in parallel with the MOSFET to clamp a voltage across the MOSFET to substantially zero. The MOSFET has a source terminal S, a drain terminal D, and a gate terminal G.
The diode of the MOSFET is turned on by a resonant operation of a resonant circuit formed by an inductor L and a capacitor C connected in parallel with the MOSFET and the diode. When a current starts to flow from the inductor L into the capacitor C during the resonant operation, the voltage across the capacitor is zero, and the diode is not conducting. As the current continues to flow into the capacitor, the capacitor charges up until the voltage across it exceeds the forward voltage of the diode, causing the diode to conduct. While the diode is conducting, the voltage drop across the diode remains at the forward voltage of the diode, which is about 0.7 V for a silicon diode, and about 0.3 V for a germanium diode, so that a voltage across the MOSFET becomes substantially zero, thereby enabling a zero voltage switching operation to be performed by a drive circuit that applies a control signal to the gate terminal G of the MOSFET to turn the MOSFET on or off. The voltage across the MOSFET remains at substantially zero until the direction of the current flow reverses during the resonant operation, causing the diode to stop conducting.
However, although the MOSFET has a fast switching speed, the design of the circuit in FIG. 1 requires the MOSFET to have a withstanding voltage, thereby increasing the cost of the MOSFET. Thus, the MOSFET is not practical for use as a switching element that requires a high withstanding voltage. Although a bipolar junction transistor (BJT) having a high withstanding voltage is less expensive than a MOSFET having a high withstanding voltage, the BJT has a slower switching speed, and thus is not practical for use as a switching element that requires a high switching speed.